1. Field of the Invention
The present invention relates to a polarization-combining fiber-fused optical coupler which, in the reverse mode, can also serve as a polarization beam splitter. The invention also relates to a method of manufacturing such coupler by splicing two polarization maintaining (PM) fibers to a pair of standard non-birefringent single mode (SM) fibers and forming a coupling zone near the splicing with the SM fibers by fusing and drawing the same until a desired power transfer is achieved.
2. Description of the Prior Art
Fused and tapered 2xc3x972 couplers, formed by fusing together sections of two laterally adjacent optical fibers and by subsequently drawing these fused sections are well known in the art. They are versatile devices that can be used in many applications. Their most common functions are optical power splitting and wavelength division multiplexing (WDM). However, WDM devices can only divide or combine optical signals of different wavelengths.
A fused-tapered fiber-optic coupler, if properly designed, can also function as a polarization beam splitter (PBS). Fiber optic PBS couplers separate light power into two orthogonal polarizations. The form birefringence, which produces a difference in the propagation constants between orthogonal polarized states, causes the polarization splitting in the coupler. A fiber optic PBS coupler is used for providing polarization sensitive devices with light signals having known polarization states.
Fiber optic polarization-combining couplers combine the power of two
Fiber optic polarization-combining couplers combine the power of two orthogonal polarizations into one single output fiber. One of ordinary skill in the art will recognize that such an optical device is bidirectional and operates in a reverse fashion from what is described above with respect to the polarization beam splitter. A polarization combiner permits to introduce beams of linearly polarized light from two light sources and combine them within a single common optical fiber.
Polarization beam-splitting properties of fused taper couplers have been known since 1985, when they were disclosed by A. W. Snyder in the article entitled xe2x80x9cPolarizing Beamsplitter from Fused-Taper Couplersxe2x80x9d, published in Electronic Letters of Jul. 4, 1985, Vol. 21, No. 14, pp623-625.
In this regard, it should be noted that a fused-fiber coupler is made of two laterally fused SM fibers, forming a structure which has two transversely symmetrical axes. Geometrically, these axes are axes of birefringence of the coupler, namely the x-axis which passes by the two cores of the fibers and the y-axis which is perpendicular to the x-axis.
The transmission of the coupler, if it is excited by one of the input arms can be represented as follows:
P1=ax2 cos2 (xcfx86x)+ay2 cos2 (xcfx86y) 
P2=ax2 sin2 (xcfx86x)+ay2 sin2 (xcfx86y) 
where P1 is the power at the output of the first arm, P2 is the power at the output of the second arm, xcfx86x and xcfx86y are accumulated phases in the coupler according to polarizations x and y, and ax and ay are amplitudes of the signal at the coupler input according to the polarization axes.
Since the accumulated phases of xcfx86x and xcfx86y are different, a coupler will have a different response according to the polarizations at the input of the coupler. Here, one can distinguish two particular situations: (1) when the accumulated phases are in phase (equal to a multiple of 2xcfx80) and have a value which is a multiple of xcfx80, the power at the input will go to one or the other output fiber; this property of matched phases is used to make multiplexers of wavelength with little dependence on polarization; (2) in the second case, if the accumulated phases are out of phase (equal to about (2n+1) xcfx80, i.e. an odd-multiple of xcfx80) and have a value which is a multiple of xcfx80, the two polarizations will be separate and each will go into a separate fiber. An example of this latter case would be, xcfx86x=mxcfx80 and xcfx86y=(mxc2x11)xcfx80, meaning that each accumulated phase is a different multiple of xcfx80. It should be noted that in all references to a multiple of xcfx80 or of 2xcfx80 or the like, the values need not be an exact multiple, but could be an approximate multiple, i.e. essentially such a multiple.
Moreover, U.S. Pat. No. 5,064,267 issued Nov. 12, 1991 to Rossberg, discloses a method of manufacturing a polarization-selective coupler, in which, during the formation of the coupling region, linearly polarized light of a selected wavelength is fed into one of the input fibers, and the coupling region is drawn-out until an equal amount of light is detected from each output fiber, at which point the source of heat is turned off, thereby stopping the elongation of the coupling region. This technique is unsatisfactory because it does not properly measure the polarization state in the coupler and, in particular, it does not measure such state at the wavelength of the operation of the coupler. Also, the technique is not sufficiently precise, as it provides for feeding only one state of polarized light into only one of the input fibers, while the other input fiber remains unused during the formation of the coupler. Thus, it only achieves a partial combining of the power, and due to this, such measurement cannot guarantee the performance of the coupler.
Furthermore, in applicant""s own international patent application published on May 25, 2001 under No. WO 01/37013, which is incorporated herein by reference, a method is disclosed and claimed for the fabrication of multiplexing and demultiplexing single-mode fiber couplers by elongating fused fibers by controllably pulling them longitudinally while heating them with a suitable heat source so as to create a coupling zone with an adiabatic profile, and stopping the elongation process when a match point between wavelength period and polarization phase is achieved. In this prior international application, it was found that it is possible to match the wavelength periods and the polarization phase at the same time. It has now been found that this principle can also be used to fabricate a polarization-combining optical coupler in accordance with the present invention.
It is an object of the present invention to create a fiber-fused polarization-combining optical coupler adapted to combine signals originating from two polarization maintaining (PM) fibers; this coupler, in the reverse mode, can also be used as a polarization beam splitter.
Another object is to produce a polarization-combining coupler that would be operational in a broad wavelength bandwidth of at least 8 nm.
A still further object is to manufacture a polarization-combining coupler that has a small insertion loss of less than 0.3 dB.
Other objects and advantages of the present invention will be apparent from the following description thereof.
According to the invention, the novel method for manufacturing the polarization-combining coupler uses the form birefringence of the fused coupler to separate the polarizations. The coupler is manufactured from standard non-birefringent single mode (SM) fibers so as to form an adiabatic tapered profile and preferably have a polarization phase difference between xcfx86x and xcfx86y along the coupler, which is essentially an odd-multiple of xcfx80, while xcfx86x and xcfx86y have values which essentially multiples of xcfx80, these conditions prevailing at a predetermined common wavelength. Polarization maintaining (PM) fibers are spliced to the input SM fibers of the coupler, preferably as close as possible to the coupling zone. The splicing can be done either before or after the formation of the coupling zone which is produced by fusion and elongation of the SM fibers. Preferably, however, splicing is done before fusion and elongation of the coupling zone, to provide better control of the operation.
The PM fibers are known to carry optical signals while maintaining polarization orientations. One of the PM fibers is oriented to maintain the polarization along the x-axis and the other along the y-axis so that one of the arms of the coupler is excited by the signal of polarization x and the other by that of the orthogonal polarization y. Because the modes of the two polarizations have a phase difference, which is an odd-multiple of xcfx80 and the accumulated phases have values that are different multiples of xcfx80, one of the polarizations remains in the same fiber as the input fiber while the other is transferred. Since the two polarizations are not injected in the same input fiber, they become combined in the same output fiber.
If a polarization splitter is desired, any polarization state, or even a depolarized signal, introduced into an SM fiber at the output end of the coupler (now serving as input fiber), will be separated into axes x and y by the coupler and transported out of the coupler by the PM fibers as separate orthogonal polarizations x and y respectively.
According to a preferred embodiment of the invention, two PM fibers are first spliced to two SM fibers. Then, the birefringent axes of the PM fibers are aligned on the x and y axes respectively of the coupler. Such alignment can, for example, be performed visually by looking at the birefringent structure in the fibers either sideways or through the ends of the fibers and properly aligning the axes. Thereafter, the SM fibers are stripped of their coating, typically over a length of 35-40 mm, and are held parallel to each other and heated and fused together. Preferably, a small degree of fusion is produced, as explained in applicant""s international application WO 01/37013. The fused portion is then drawn while being heated to constitute a tapered shape with an adiabatic profile, thereby producing a coupling zone. This coupling zone is normally produced as close as possible to the PM/SM fiber splices, usually at a distance in the order of 20 mm.
The spectral response of a fused fiber coupler is oscillatory since there are multiple power exchanges between the two fibers along the length of the coupler. The operation of the polarization beam combiner in accordance with this invention relies on an effect associated with the form birefringence exhibited by the fused section of the coupled device. Said form birefringence can be controlled by correspondingly monitoring the temperature of the fibers being drawn. The drawing end point is determined by controlling the power signal at the two output SM fibers of the coupler. Linearly polarized light is fed from the PM fibers, one at a time, into each of the input arms of the coupling zone, and the transmitted power is detected at the output fibers following the coupling zone. The light source and detector combination can be, for example a polarized broadband source and a spectrum polarizer or a tunable laser and a detector, so that the wavelength properties of the couplers can be measured simultaneously with the polarization properties. A maximum power transmission must be found in one of the output fibers at a desired wavelength for the linearly polarized light inputted in each of the two input arms of the coupling zone by the PM fibers, while keeping the insertion loss at a minimum. The drawing process is terminated when the coupling of the output powers in one of the output fibers reaches maximum for both polarizations at a desired wavelength. Said desired wavelength is achieved by controlled drawing of the fibers. This point also corresponds to a minimum transmission to the other unused output fiber.
To summarize, the polarization-combining fused-fiber optical coupler of the present invention comprises a coupling zone made of two non-birefringent single-mode (SM) fibers fused and drawn so as to form an adiabatic tapered profile, this coupling zone having two input arms and two output fibers; and two polarization maintaining (PM) fibers are spliced to the input arms of the coupling zone; the coupler being characterized in that when linearly polarized light is injected into each PM fiber and oriented so that polarization in one PM fiber is orthogonal to that of the other PM fiber, and when this light passes through the input arms into the coupling zone, it combines maximum transmitted power of the two polarizations at a desired common wavelength in one output fiber, with only a minimal power transmission going to the second output fiber. Preferably said coupling zone is so shaped as to have, at the desired common wavelength, a polarization phase difference between accumulated phases xcfx86x and xcfx86y of a value that is essentially an odd-multiple of xcfx80, while xcfx86x and xcfx86y have values which are essentially multiples of xcfx80.
The method of manufacturing a polarization-combining fused-fiber optical coupler in accordance with the present invention comprises:
(a) taking two polarization maintaining (PM) fibers having birefringent axes and each having a light input and a light output end;
(b) splicing said polarization maintaining (PM) fibers by their output ends to two non-birefringent single mode (SM) fibers;
(c) aligning the birefringent axes of the PM fibers at 90xc2x0 of each other, one being aligned with the x-axis and the other with the y-axis of the coupler;
(d) injecting linearly polarized light into a same birefringent axis of each PM fiber so that polarization in one PM fiber output end is orthogonal to that in the other PM fiber output end; and
(e) fusing and drawing the two SM fibers near the splicing, to form a coupling zone, the drawing being carried out so as to form an adiabatic tapered profile and is stopped when maximum power transmission at a desired common wavelength is detected in one of the two output SM fibers which extend from the coupling zone.
Preferably, fusing and drawing of the SM fibers is performed so as to produce a polarization phase difference between accumulated phases xcfx86x and xcfx86y which is essentially an odd-multiple of xcfx80, while xcfx86x and xcfx86y have values which are essentially multiples of xcfx80.
As already mentioned previously, this invention is not limited to the making of a polarization combiner. It is obvious to a person skilled in the art that this device may also be used as a polarization splitter. In such a case, instead of monitoring power transmission, polarization analysis is needed at the output of the device. The drawing end point of the coupler is reached when the orthogonal polarizations of an input beam are split between the two output ports and thus the coupler acts as a polarization beam splitter.